1. Field of the Invention
The present invention is directed to a method and to an apparatus for determining the volume of a bellows reservoir for medication in an implantable infusion system.
2. Description of the Prior Art
In an implantable infusion system, wherein medication to be dispensed is contained in a reservoir, there is a need to continuously determine the volume of medication in the reservoir. This is for at least two reasons, first, it must be ensured that the medication volume in the reservoir does not become too low before a medication refilling procedure is undertaken, otherwise air, which is always present in the reservoir, would then be pumped into the patient. Second, because the pressure in the bellows reservoir decreases as the medication volume becomes less than 2-3 ml, if the medication volume reaches such a low level the pump must operate with a larger pressure difference, resulting in reduced medication volume per pump stroke, leading to cavitation in the pump. Such cavitation is detrimental to medications like insulin, whose molecules are "fragile."
Moreover, it is desirable to be able to determine the volume of the medication in the bellows reservoir during a filling procedure, in order to avoid overfilling the reservoir. The bellows reservoir is contained in the implantable unit in a chamber which creates appropriate pressure conditions for siphoning medication from a refilling apparatus which is temporarily placed in fluid communication with the implanted unit during the refilling procedure. If the bellows reservoir is overfilled to the extent that the bellows wall comes into contact with the wall of the chamber, the conditions for siphoning are no longer present.